The present invention relates to electrical contact system and an electrically conductive filter element comprising such electrical contact system.
Electrically conductive filter elements are known, e.g. from EP0764455. A filter element, comprising metal fibers, is used to filter soot particles. To regenerate the filter element, an electric current is applied. The filter is heated by the Joule effect.
At present, the electric current is supplied to the filter fleece via electrical contact bodies, which are welded to the fleece, or which may be clamped to the fleece.
In spite of all care, the contact surface between contact body and fleece has some disadvantages. Due to the high current during welding, the fleece may be damaged e.g. burned locally. When current passes from contact body to the fleece, the current tends to follow preferential routes. The current flows preferentially from contact body into the fleece at the places, where, due to uneven welding, the contact between contact body and fleece is most intimate, compared to the rest of the contact surface between contact body and fleece. The same happens when contact body and fleece are clamped to each other.
These preferential routes cause so called xe2x80x9chot spotsxe2x80x9d on the fleece surface. These spots tend to heat up more than the rest of the fleece surface, since more current flows locally, and the temperature is increased locally extensively due to the Joule effect. At these hot spots, the fiber fleece will oxidize more, causing quick degeneration of the fleece locally. This finally results in a burning through of the transition zone between the fleece and the contact body.
This problem occurs not only for electrically regenerating filters, but also occur in all cases where a contact body (having a low electrical resistance) is to contact an electrically conductive porous fleece (having a high electrical resistance).
The invention has the objective to provide an electrical contact system, which comprises a contact body and a electrically conductive porous fleece, which does not have these disadvantages. According to the present invention, the electrically conductive porous fleece and the contact body are sintered to each other.
It was found than, when the contact body and the electrically conductive porous fleece are sintered to each other, no hot spots were noticed during the supply of electric current to the electrically conductive porous fleece via the contact body. Due to the very intimate and equal contact over the total contact surface between contact body and electrically conductive porous fleece, the electric current which has to flow from contact body to the electrically conductive porous fleece will meet the same resistance on every spot of the contact surface between contact body and electrically conductive porous fleece.
The electric current, provided to a first contact body by an electrical circuit, will flow from this contact body, via a contact surface into the electrically conductive porous fleece. The electric current further flows through the electrically conductive porous fleece towards a second contact body, also making contact to the electrically conductive porous fleece via a contact surface. It is clear that, in the scope of the invention, the electric current flowing through the electrically conductive porous fleece effects the heating of the filter element.
In an electrical contact system as subject of the invention, the electric current does not flow from contact body into the electrically conductive porous fleece via preferential routes. The change from low electrical resistance (contact body) to higher electric resistance (electrically conductive porous fleece) will be very smooth and an equal current distribution over the total volume of the electrically conductive porous fleece is provided. Due to the sintering action, very intensive contact is obtained between fibers of the electrically conductive porous fleece and the contact body. They are so to say micro-welded to each other.
With electrically conductive porous fleece is meant a preferably sheet-like volume of electrically conductive fibers and/or particles. Preferably, metal fibers or metal particles are used. Most preferably, steel fibers with a high specific electrical resistance are used. Examples of fibers are stainless steel fibers from AISI 300- or AISI 400-serie alloys such as AISI 316L or AISI347, or alloys comprising Fe, Al and Cr, such as Fecralloy(copyright). Preferably stainless steel comprising chromium, aluminum and/or nickel and 0.05 to 0.3% by weight of yttrium, cerium, lanthanum, hafnium or titanium are used.
Fiber diameter preferably is between 1 and 100 xcexcm, e.g. between 4 and 25 xcexcm. The electrically conductive porous fleece is connected to one or more contact bodies by a sintering action. So the electrically conductive porous fleece will become a sintered fleece. However, an electrically conductive porous fleece may be already sintered before it is subjected to a sintering action to connect it to one or more contact bodies.
The metal fibers may be obtainable by bundle drawing or by shaving techniques (e.g. as described in U.S. Pat. No. 4,930,199), or by any other process as known in the art.
With contact body is meant an object which is connected to an electric circuit, e.g. via a lead wire, and which is connected to the electrically conductive porous fleece via a contact surface in order to provide the electric current from the electric circuit to this electrically conductive porous fleece.
According to the invention, a contact body may e.g. be a metal volume, an expanded or perforated metal plate, a woven metal wire net, a woven or knitted metal fiber fabric or a metal foil, or even metal fiber yarn which extends in a flare. The contact body contacts the electrically conductive porous fleece over a limited surface.
According to the invention, one or more contact bodies are sintered to an electrically conductive porous fleece. The position of these contact bodies on the electrically conductive porous fleece may be changed depending on the requirements as specified by the use of the electrical contact system. Usually, two or more contact bodies are sintered at the border of the electrically conductive porous fleece. Most usually, the contact bodies are so sintered to the electrically conductive porous fleece that the largest distance between the contact bodies is provided.
In case the contact bodies are metal foils, metal mesh, wires metal yarns or woven, braided or knitter metal fabrics, the contact bodies usually extend the border of the electrically conductive porous fleece to which they are sintered. Most usually, they are sintered to one side of the electrically conductive porous fleece, or they are located between two layers of the electrically conductive porous fleece. To obtain this, a first layer of electrically conductive porous fleece is provided. The contact bodies then are positioned on the surface of this first layer of electrically conductive porous fleece. They are positioned in such a way that they partially extend a border of the electrically conductive porous fleece. A second layer of electrically conductive porous fleece is put on the first layer and the contact bodies. These elements are then sintered together in appropriate circumstances. An electrical contact system is so obtained, comprising a electrically conductive porous fleece, out of which contact bodies extend.
Alternatively contact bodies are clamped to the border of the electrically conductive porous fleece, before they are subjected to a sintering action.
In case metal yarn, comprising metal fibers or filaments, is used to provide contact bodies, the fibers or filaments which extend from the yarn at the end of the yarn which is to be sintered to the electrically conductive porous fleece, are unraveled to some extend, providing a flare. This flare, or so to say brush-like structure, is then sintered to the electrically conductive porous fleece, so increasing the contact surface between contact body and electrically conductive porous fleece.
Preferably, the contact body and the electrically conductive porous fleece, comprising metal fibers, are provided by the same metal alloy.
Such electrical contact system as subject of the invention may be used to provide a heating element or a heatable or regeneratable filter, e.g. an exhaust particulate filter, such as a diesel exhaust filter. According to the functionality, required by the application, the electrically conductive porous fleece may have a different porosity, thickness, composition, electrical resistance, dimension or shape. Also according to the application, the position of the contact bodies compared to the electrically conductive porous fleece may be chosen.